docs: def _handle_result(self):
          """Process one result. Block untill one is available
          """
          result = self.inbox.get()
          if result.success:
              if self._verbosity >= VERB_PROGRESS:
                  sys.stderr.write("\nuploaded chunk {} \n".format(result.index))
              self.results.append((result.index, result.md5))
              self._pending_chunks -= 1
          else:
              raise result.traceback